Physical activity induces a subclinical inflammatory response, mediated in part by leukocytes, and manifested by elevated concentrations of circulating proinflammatory cytokines, including interleukin (IL)-1Beta, IL-6, and tumor necrosis factor-alpha (TNF-alpha). However, the source of the cytokines that appear during exercise remains unknown. In this study, we examined exercise-induced changes in plasma cytokine concentrations and their corresponding mRNA expression in peripheral blood mononuclear cells. Then healthy (peak oxygen uptake = 48.8 + or - 6.5 (SD) ml-kg 1(-).min 1(-) but untrained men (age = 25 + or - 5 (SD) yr) undertook 3 h of exercise (cycling and inclined walking) at 60-65% peak oxygen uptake. Circulating leukocyte subset counts were elevated during and 2 h postexercise but returned to normal within 24 h. Plasma concentrations of IL-1Beta, IL-6, and TNF-alpha peaked at the end of exercise and remained elevated at 2 h (IL-6) and up to 24 h (IL-1Beta and TNF-alpha) postexercise. Cytokine gene expression in circulating mononuclear cells was measured by using the reverse transcriptase-polymerase chain reaction; mRNA accumulation did not change with exercise. TRUNCATED